1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus.
2. Description of the Related Art
Optical scanning apparatuses are used in image forming apparatuses, such as an optical printer, an optical plotter, an optical facsimile apparatus, and a digital copying machine.
With the progress of colorization of these image forming apparatuses, a so-called tandem type color image forming apparatus has become very popular to increase the image forming speed. The tandem type color image forming apparatus forms toner images of different colors respectively on a plurality of photoconductive surfaces, and sequentially transfers the toner images one on another on a sheet-like recording medium, thereby forming a color image. An example of such conventional technology is disclosed in Japanese Patent Application Laid-open No. 2002-90672.
Further, to achieve high-quality, high-definition images with the optical scanning apparatus, the spot size of a light beam spot is made smaller.
The spot size of a light beam spot on a surface to be scanned (hereinafter, referred to as scanning target surface) is determined by the design of the optical system that constitutes the optical scanning apparatus. Theoretically, a light beam spot is accurately formed on the scanning target surface, i.e., when the light beam waist position of a light beam that forms a light beam spot matches with an image surface. In practice, there are factors that shift the light beam waist position with respect to the scanning target surface.
One of such factors is deformation of the shape of the deflective reflection surface of a polygon mirror used as a light beam deflector, caused by the high-speed rotation of the polygon mirror. That is, the polygon mirror, which is generally formed of a light metal like aluminum, rotates fast at a very high speed of, for example, 20,000 rotations per minute (rpm). The high-speed rotation causes large centrifugal force to act on the polygon mirror, thus deforming the deflective reflection surface.
While the deflective reflection surface is normally flat, it is curved when deformed by the centrifugal force. In this case, in the optical system constituting the optical scanning apparatus, the deflective reflection surface itself will have an image forming action although small, thus changing the light beam waist position or the image-forming position of a deflected light beam. This enlarges a spot size such that the size of the light beam spot becomes greater than the designed size, thereby reducing the resolution of a formed image.
Methods of compensating for the influence of the deformation of the deflective reflection surface of a polygon mirror are proposed in the specifications of Japanese Patent Nos. 2746397 and No. 3432054.
The method disclosed in Japanese Patent No. 2746397 is based on the fact that a laser beam from a light source has a Gaussian intensity distribution on the cross section of the light beam, and reduces or prevents the enlargement of a spot size by adjusting the emission intensity of the light source. The method disclosed in Japanese Patent No. 3432054 designs the scanning optical system to cancel out a deviation of the light beam waist position caused by the deformation of the deflective reflection surface of the polygon mirror.
Japanese Patent No. 2746397 discloses a case in which the deflective reflection surface is deformed into a concaved surface, and Japanese Patent No. 3432054 discloses a case in which the deflective reflection surface is deformed into a convex surface.
The deformation of the deflective reflection surface according to the high-speed rotation of the polygon mirror, and the degree of the deformation actually depend on various factors, such as the number of rotations of the polygon mirror, the number of the deflective reflection surfaces, the shape of the deflective reflection surface, and the diameter of the rotational axis, and can fluctuate among polygon mirrors manufactured with the same design.
The method described in Japanese Patent No. 3432054 is effective when the amount of deformation of the deflective reflection surface is specifically known beforehand. However, in reality, the amount of deformation of the deflective reflection surface fluctuates according to various factors, and is not uniquely specified. When the scanning optical system is designed to eliminate the influence of the deformation of the deflective reflection surface, the design of the scanning optical system is changed every time a parameter like the number of rotations on the polygon mirror is changed.
The method described in Japanese Patent No. 2746397 requires an electrical unit that regulates the intensity of the laser beam source according to the deformation of the deflective reflection surface.